1. Field of the Invention
The present invention relates to a method to measure optical input power by an avalanche photodiode (hereafter denoted as APD)
2. Related Prior Art
The United States patent, the U.S. Pat. No. 6,188,059, has disclosed a light receiving circuit for an APD including two stages of the current mirror circuit to monitor a photocurrent generated by the APD. This circuit may detect the photocurrent of the APD without affecting a bias condition of the APED by the two stages of the current mirror circuit. The Japanese Patent Application published as JP-2004-289206A has disclosed a circuit for monitoring a light intensity using an APD. Obtaining a characteristic regarding to the multiplication factor M of the APD, which depends on the optical input power, in advance to the practical application, the monitored value by the APD shows a linear dependence on the optical input power. The United States patent, the U.S. Pat. No. 7,761,013, has disclosed a circuit for an APD. The circuit disclosed therein may select one of circuits depending on the magnitude of the photocurrent by the APD, which may widen the dynamic range of the monitoring.
An optical receiver, in particular, an optical received implemented with an APD is necessary to detect an optical signal with an intensity thereof nearly equal to a minimum condition defined in the optical system precisely. However, an APD inherently shows a dark current depending on a bias voltage applied thereto and a temperature thereof; accordingly, the dark current of the APD often hides the optical input power included in the photocurrent. Thus, an erroneous detection of the photocurrent sometimes causes the optical system in a failure.
A precise monitoring of the optical signal assumes a linear relation between the output current from the APD and the photocurrent due to the optical signal. However, as described, the dark current of the APD becomes substantial for an optical input with faint power. The methods disclosed in the prior arts above are hard to detect faint photocurrent with enough preciseness.